The study area is the N'zo-Sassandra watershed (Ivory Coast) located between longitudes 7°15' and 8°05' West and latitude 6°50' and 7°50' North. Its surface is estimated at 4310 km2. The climate regime prevailing at the N'zo-Sassandra watershed at Kahin is a mountain climate (subequatorial regime) with annual rainfall amounts sometimes exceeding 2000 mm. N'zo-Sassandra has a single hydrological regime. The N'zo-Sassandra watershed is a part of the West forest zone characterized by a degraded dense forest. This study aims to identify trends in the rainfall-runoff relationship from an overall conceptual modeling monthly runoff. The data (rainfall, ETP, flow) used in this study cover the period of 1980-2000. The methodological approach is based on the one hand, on the evaluation criteria of the model used (GR2M) and secondly, on the crossed simulations method (1980-2000). The results showed that the GR2M model was efficient and robust on the N'zo-Sassandra watershed. Indeed, the values of the Nash-Sutcliffe obtained are greater than 60% in the calibration phase (81.95%) as validation phase (72.40%). Degradation recorded at the Nash-Sutcliffe criterion passing the validation phase in the calibration phase is less in absolute value than 10%. In addition, it is shown that there is a vast majority of the positive signs in relation to the negative signs. These results highlight the existence of non-stationarity of the rainfall-runoff relation at the N'zo-Sassandra watershed during the period 1980-2000, suggesting a rising trend of monthly runoff.